It is often very important to know what fluids are flowing through a conduit such as a pipeline. For example, a buyer and seller may agree upon a price for the fluid flowing through a process pipeline based upon the content of the fluid stream. Thus, the fluid content must be measured. Where multiple pipelines are positioned near one another, it may be economical to use a single meter or measurement device to monitor all of the fluid flows. The device used to extract and deliver the fluid to the measurement device is traditionally referred to as a sampling system.
FIG. 1 includes a stream sampling system (“sampling system”) 100. Although only a single pipeline is shown, it is to be understood that multiple pipelines may be present. Sampling system 100 includes a sample point attached to pipeline 110, an analyzer 130, and tubing 120 from the sample point to the analyzer 130. Analyzer 130 may include a stream switching system 140 and gas chromatograph 150. In operation, fluid flow through a process pipeline 110. The sample point (preferably a probe fluid) obtains a sample of fluid and delivers it to analyzer 130 via tubing 120. Analyzer 130 measures the content of the fluid sample and either returns the sample to the pipeline or vents the sample to the ambient environment.
One problem with such a layout is the large distance from the analyzer 130 to the pipeline 110, which creates a large “dead volume” of fluid. Increased dead volume results in undue mixing of consecutive fluid samples. This mixing of fluid samples results in “carry-over” between samples for gas chromatograph analysis. Carry-over is undesirable because accurate analysis requires that the analysis is representative of the fluid in the process pipeline. Since the volume of transport tubing and stream sampling components must be flushed a minimum of ten times to insure a representative sample, the “dead volume” results in significant lag time between sample analysis. Therefore, upon a sampling of fluid from the pipeline 110, the “dead volume” of fluid must be vented or otherwise disposed of before the new sample can be measured at the analyzer 130. Further, although the magnitude of the “dead volume” could be reduced by placing the analyzer 130 closer to the sample point 110, regulations and safety concerns mandate a minimum 50 feet distance between them. If placed closer than 50 feet from the pipeline 110, the analyzer 130 must be contained in an expensive explosion-proof housing.
FIG. 2 includes a stream switching system 140 attached to an analyzer oven 250 that is part of gas chromatograph 150. Three pipes or tubes 210, 220, 230 attach to switching system 140, and correspond to first, second and third flow paths. The first pipe or tube 210 connects to a first sample point 212 and carries a first gas stream of unknown composition from, for example, a process pipeline. Included along “stream 1” are pressure regulator 214 and pressure gage 215, shut-off valve 216, particulate filter 217, and a first stream switching valve 218. Second pipe or tube 220 connects to a second sample point 222 and carries second gas stream of unknown composition. Included along “stream 2” are pressure regulator 224 and pressure gage 225, shut-off valve 226, particulate filter 227, and a second stream switching valve 228. The third pipe or tube 230 connects to a third sample point 232 and a calibration sample of known composition. Included along the third path are pressure regulator 234 and pressure gage 235, shut-off valve 236, particulate filter 237, and a third switching valve 238. Third switching valve 238 connects not only to filter 237, through one port, but also to first and second switching valves 218, 228 through another. Yet another port of third switching valve 238 attaches to regulator 240 and flow meter 245. Flow meter 245 attaches through a relatively long tube to sample shut-off valve 255 housed in analyzer oven 250. Sample valve 255 connects to a sample valve in the oven, and then connects to the vent 260. As can be appreciated, although only two streams of unknown fluids are shown, additional streams could be added by the use of a greater number of flow paths.
During operation, a gas chromatograph housed in analyzer oven 250 is calibrated using the calibration sample from sample point 232. The pressure and flow rate of this stream are maintained by pressure regulator 234, regulator 240 and flow meter 245. Because the composition of the calibration sample is known, it may be used to calibrate the gas chromatograph. The calibration sample flows through third switching valve 238, through the gas chromatograph 150 and out sample vent 260. If a measurement of the fluid at sample point 222 is desired, the gas along the second pipe is allowed to flow by actuation of second stream switching valve 228, through first stream switching valve 218, and through third stream switching valve 238. The third switching valve 238 is the only valve in the stream switching system that on its own can prevent or block the flow of fluid from all the sample points. Thus, this configuration is referred to as a “single block” stream switching system. One drawback of this design is that the fluid from sample point 222 flows through all of the first, second, and third switching valves prior to arrival at the gas chromatograph, and malfunction of only a single one of these switching valves prevents the measurement of a sample of fluid from stream 2.
If, after the above-described measurement of stream 2, it is desired to measure the fluid from stream 1, the system must be purged of the previous fluid sample. Purging of the old fluid stream from the system prevents contamination between the streams. Thus, the stream switching system of FIG. 2 would switch from stream 2 to stream 1. At that time, adequate accuracy by the gas chromatograph has likely been assured if all the other necessary criteria have been met. Many refer to a configuration having a single sample vent as a “single bleed” stream switching system.
Thus, a “dead volume” of fluid in a stream switching system is a significant problem. Another problem encountered in a stream switching system is the reliability and maintenance of the system. Because an operator may visit a particular stream switching system only infrequently, the system should be accurate, reliable, as immune to breakdown as possible, and simple to repair when problems do occur. This highly sought after combination of features is not available with current stream switching systems.
Another drawback in many prior systems is their difficulty in analyzing a complex fluids because of limitations in the associated gas chromatographs. It would be desirable if a stream switching system could be developed that could quickly transfer fluid sample to the analyzer. This drawback also reduces the usefulness of a stream sampling system.
A stream sampling system is needed that is faster, more reliable, more flexible, and more accurate than previous stream sampling systems. Ideally, such a stream sampling system could reduce the adverse effects of “dead volume.” This ideal stream sampling system would also be less prone to breakdown than previous models, while providing much faster and more accurate measurements.